There are many membranes which to a certain degree possess the property of being selectively permeable to various components of solution mixtures. Thus, for example, certain membranes exhibit retentivity toward ions, while allowing water to pass through. Other membranes have selectively different diffusion rates for two or more different nonionic components, while still other membranes are of the so-called molecular sieve type. Such properties are widely applicable, for example for recovering water from saline solutions, as in the desalination of seawater, water softening or purification of wastewater, recovery of small amounts of dissolved or colloid-disperse substances from solutions, concentration of solutions or dispersions, or separation or purification of macromolecular or colloidal materials from solutions which contain contaminants with low molecular weights. In the latter case, for example, the purification of blood and use in artificial kidneys are especially well-known examples.
An area of application which is particularly important for industrial technology is the desalination of seawater and brackish water to obtain drinking water. Membranes made of completely synthetic polymers, especially polyamide hydrazide membranes, membranes made of mixtures of cellulose diacetate and triacetate, as well as multilayer membranes composed of a cellulose ester carrier membrane coated with cellulose triacetate, have been proposed and used. Seawater contains approximately 35,000 ppm dry substance and/or salt; drinking water may contain a maximum of 500 ppm of salt. In order to clean water sufficiently in one pass through the membrane, a theoretical salt rejection (R) of 98.6% would be necessary. In practice, however, due to certain invariable loss factors, a salt rejection (R) of 99.5% is required. Salt rejection R, however, is not the sole criterion for such a membrane. The membrane must also exhibit satisfactory diffusivity (D) in order to be technically usable. The lower economic limit is D= 400 liters/m.sup.2 /day.
Known membranes mostly suffer from the fact that they have excessively low diffusivity and high salt rejection or vice versa. German Auslegeschrift No. 15 70 163 (Loeb) teaches an acetate membrane, but it does not have sufficient diffusivity. By way of improvement, German Auslegeschrift No. 21 15 969 teaches membranes made of cellulose-2,5-acetate, which have a diffusivity of 100 liters of water per m.sup.2 of membrane surface in 24 hours for salt water with an NaCl content of 5000 ppm (0.5%), while the salt content of the water which has passed through is still 600 ppm (0.06%), (R= 88%). With higher salt rejection, in order to obtain a permeate with 100 ppm (0.01%), (R= 98%) of salt, diffusivity is only 40 liters per m.sup.2 per day. Hence, the permeate must be passed repeatedly through a membrane in order to achieve sufficient desalination; in other words, it must operate in a "multistage" mode. Therefore, it has been stated, for example, in a publication (4th International Symposium on Fresh Water from the Sea, Vol. 4, 285-295, 1973) that by using known membranes, only brackish water with a salt content of approximately 1% can be turned into drinking water. With higher initial concentrations, either the efficiency, i.e., the diffusivity or the retentivity (salt rejection) of the membranes is too low.
Single-stage seawater desalination is more economical, however, than the multistage type and would therefore be of greater engineering significance.
In engineering applications, cellulose diacetate is used, although it is well known that cellulose triacetate should theoretically be better suited for desalination (Riley et al., 3rd International Symposium on Fresh Water from the Sea, Vol. 2, 551-560, 1970). It has the advantage of greater resistance to hydrolytic and biological attack than diacetate. The importance of resistance to hydrolysis becomes apparent in view of the fact that the pH of sea-water is approximately 8.5, and diacetate membranes can only be used following acidification to pH 6. The necessity of prepariing, storing, and dispensing sulfuric acid is especially disadvantageous when the point at which it is to be used is remote. Thus, in an intensive trial of alkaline hydrolysis at ph 11.5 and 50.degree. C., a triacetate film shows a decrease in acetyl content (based on 100% acetyl content in the original material) to only 37.9% after 4 hours, while a diacetate film shows a decrease in acetyl content from 100% to 5.8%.
However, it has thus far been possible only to use cellulose triacetate mixed with cellulose diacetate in so-called mixed membranes (membranes of the blend type) or as a thin coating layer on multilayer membranes. The latter are very costly to manufacture, however.
The production of a skin layer with cellulose triacetate has so far been unsuccessful because cellulose triaceate is difficult to work with, soluble only in a few solvents, and the solutions have high viscosity. Moreover, it is known (German Auslegeschrift Nos. 15 70 163 and 21 15 969) that membranes made of higher-substituted acetate are much less permeable than those made of diacetate, so that the diffusivity is too low. The desalination efficiency of cellulose acetate admittedly increases with the degree of acetylation (Riley, et al.), but the permeability to water decreases markedly as well. One consequence of the low permeability of cellulose acetate in general to water, therefore, is the requirement for an extremely thin, active layer (skin layer) with the membranes to achieve efficient diffusivity.
Conventional membrane filters made of cellulose diacetate do not exhibit any useful desalination properties and must therefore be tempered in water at 70.degree.-90.degree. C. Tempering, however, results in a significant decrease in diffusivity. Direct to of this tempering process for diacetate in water in triacetate membranes has not yet been possible.
So-called integral membranes, in other words, membranes made of only one layer, but one which is sufficiently thick to be easy to handle, would be much more satisfactory than multilayer membranes, however. Integral membranes are simpler to manufacture. They can be prepared essentially in a single casting from only one homogeneous material. Owing to the density of triacetate membranes, which does not allow sufficient diffusivity, and owing to the additional difficulty that no success has yet been achieved in tempering triacetate membranes to improve their properties sufficiently, no integral triacetate membranes have yet been used.
The membrane filter most widely used at present, as mentioned above, is a diacetate filter, made by the so-called Loeb method. Experiments aimed at transferring the Loeb method to cellulose triacetate have yielded unsatisfactory results. Skiens and Mahon (German Offenlegungsschrift No. 19 23 187) and J. Appl Poly. Sci., Vol. 7, 1549 (1963) achieved a diffusivity of 57 liters/m.sup.2 /day with a salt rejection of 92.5% (on the basis of 1% NaCl) (48.9 bars). The most satisfactory results described in the literature (Saltonstall, 3rd Internation Symposium on Fresh Water from the Sea, Vol. 2, 579-586, (1970)) with triacetate membranes indicate diffusion of 180 liters/m.sup.2 and salt rejection of 98.7% with an initial salt concentration of 3.5% NaCl and 105 bars pressure differential. The membranes were drawn from acetone/dioxane with methanol and maleic acid additives; it is also claimed that mixtures of diacetate and triacetate are preferable to the pure components. In contrast to the Loeb method, for example, cellulose triacetate membranes can also be manufactured by melt extrusion with sulfolane and polyethylene glycol. However, unsatisfactory results have resulted from using this procedure (Davies et al., ACS Polym. Prepr., 12, (2), 378 (1971)).
All of the membranes described thus far, made of pure cellulose triacetate, are inferior to Loeb membranes based on cellulose diacetate.
The principal difficulty regarding triacetate is the limited choice of solvents and the very high solution viscosities in the required concentration range (14-18%). These become particularly evident when, by analogy with known processes, one of the highly volatile water-miscible solvents (dioxane, tetrahydrofuran) is used instead of acetone.
The only principal component of all known casting solutions for manufacturing cellulose acetate membranes of the Loeb type that has been found satisfactory thus far is acetone. The use of large amounts of acetone appears to be a prerequisite for the Loeb method.
Since cellulose triacetate is insoluble in acetone, other solvents must be added to cellulose triacetate casting solutions. The amounts required for this purpose are a function of the degree of polymerization of the triacetate; the higher the molecular weight, the more actual solvent required (e.g. dioxane, dimethyl sulfoxide). According to a known method, dioxane:acetone ratios much larger than 1:1 are used (King, Hoernschemeyer and Saltonstall in "Reverse Osmosis Membrane Research", Plenum Press, New York-London, 1972, p. 148).